Add $options bitmask arg to EventLogging::logEvent

Adds an optional argument to EventLogging::logEvent, $options, set to 0 by
default. It is a bitmask which may contain EventLogging::OMIT_USER_AGENT,
indicating the user-agent header should not be logged.

Change-Id: Id23b37fbe6c76afabb4b54ade744d1d436e5c15d